There is known, in particular from U.S. Pat. No. 6,185,158, an electronic watch fitted with an analogue display for several time parameters, in particular the hours, minutes and seconds, by means of three coaxial hands at the centre of the watch dial. Further, the analogue display includes a chronograph hand, in particular a minute hand for the measured time interval, associated with a circular graduation over 360°, and a date display utilising a date ring, the displayed date appearing in conventional manner through an aperture in the dial. This Patent proposes to actuate the mechanism driving the chronograph hand (hereafter the “first mechanism”) and the mechanism driving the date ring (hereafter the “second mechanism”) via one and the same electromechanical motor. In the embodiment described, the first mechanism must not be associated with the display of the current hour, minute or second or of another function connected to these time parameters, whereas the second mechanism must be of the intermittent drive type, as is the case for the date display.
The first mechanism includes an intermediate wheel driven directly by the rotor of the motor and a chronograph wheel that meshes with the intermediate wheel. The second mechanism also includes said intermediate wheel and also an auxiliary wheel meshing with said intermediate wheel. The auxiliary wheel is integral with a wheel set that periodically actuates a wheel driving the date ring, this wheel set having a finger for actuating the drive wheel. The periodic actuation wheel set and the drive wheel together form a Geneva mechanism, known for periodically driving a date ring/disc. On each revolution of the periodic actuation wheel set, the finger drives the date ring drive wheel, which is driven in rotation over an angular distance corresponding to the change from one date to the next in the aperture in the dial provided for the date display. The Geneva mechanism is thus characterized by the periodic driving of the date ring drive wheel, with the periodic actuation wheel set only meshing with the drive wheel over an angular sector of less than 360°, whereas the wheel set locks the drive wheel on the remaining angular sector. Thus, although the periodic actuation wheel set rotates when positioned in the remaining angular sector, the rotational motion of the rotor is not transmitted to the date ring.
U.S. Pat. No. 6,185,158 uses the Geneva mechanism to enable the motor used for driving the date mechanism to perform an additional function, namely driving a chronograph hand. In short, the method consists in driving the chronograph hand when said periodic actuation wheel set is in its area of non-actuation, i.e. in said remaining angular sector, and, at the end of the measured time interval, in performing a reverse reset to return the periodic actuation wheel set to a predefined initial position. To achieve this, the chronograph hand is arranged to make only one revolution and is driven such that one complete revolution leaves the periodic actuation wheel set in its area of non-actuation. In fact, the seconds hand and the chronograph minute hand are arranged to be used in chronograph mode. Thus, there is no counter indicating the number of revolutions made by the chronograph minute hand, so that the maximum time interval able to be measured corresponds to a single revolution of the chronograph hand concerned. Stopping the chronograph within this maximum time interval or at the end thereof, ensures that the date disc is never driven.
The method for controlling the analogue display device for a chronograph hand and date according to the aforementioned prior art raises at least two major objective problems. First of all, this method is essentially limited to a chronograph hand displaying the highest time unit that can be counted by the chronograph display. Indeed, if it is desired to introduce an hour counter (for example up to three hours) in addition to the 10 minute counter provided (FIG. 1), the control method proposed is no longer suitable since said periodic actuation wheel set is then liable to make eighteen revolutions over the maximum time interval that can be measured; which, in the example set out in U.S. Pat. No. 6,185,158 would change the date four times during the measured time. Secondly, the date ring is inadvertently driven if a user forgets to stop the chronograph substantially within the maximum time interval provided (ten minutes) or at the end thereof. Of course, the area of non-actuation provided is sufficient to allow the minute counter to complete three revolutions before actuating the date ring. However, if the user forgets to stop the chronograph function for any reason, hours may pass before he or she realises. In that case, the date display will be changed in error. Further, the chronograph mode may be unintentionally started, which leads to the same result.